New "DIV.ROC" option corporate action activity sub-type

Adds DIV.ROC (Return of Capital) to the OPCA option corporate action activity sub-types.

• activity_sub_type on account activities can now return DIV.ROC for option positions.
• The option cash dividend activity schema now covers both cash dividend and return of capital events.

Broker API

• GET /v1/accounts/activities
• GET /v1/accounts/activities/{activity_type}
• GET /v2beta1/accounts/{account_id}/events/activities/{event_id}
• GET /v2beta1/events/activities

Trading API

• GET /v2/account/activities
• GET /v2/account/activities/{activity_type}
• GET /v2beta1/events/activities